Mohsen Alishahiha

Supergravity and large N noncommutative field theories

We consider systems of Dp-branes in the presence of a nonzero B field. We study the corresponding supergravity solutions in the limit where the branes worldvolume theories decouple from gravity. These provide dual descriptions of large N noncommutative field theories. We analyse the phase structure of the theories and the validity of the different description. We provide evidence that in the presence of a nonzero B field the worldvolume theory of D6 branes may decouple from gravity. We analyse the systems of M5 branes and NS5 branes in the presence of a nonzero C field and nonzero RR fields, respectively. Finally, we study the Wilson loops (surfaces) using the dual descriptions.

Non-supersymmetric attractors and entropy function

We study the entropy of non-supersymmetric extremal black holes which exhibit attractor mechanism by making use of the entropy function. This method, being simple, can be used to calculate corrections to the entropy due to higher order corrections to the action. In particular we apply this method for five dimensional non-supersymmetric extremal black hole which carries two magnetic charges and find the R2 corrections to the entropy. Using the behavior of the action evaluated for the extremal black hole near the horizon, we also present a simple expression for C-function corrected by higher order corrections.

Charged black branes with hyperscaling violating factor

A bstractWe present an analytic solution of a charged black hole with hyperscaling violating factor in an Einstein-Maxwell-Dilaton model where the scalar potential is key to the existence of a solution. This solution provides a candidate gravitational description of theories with hyperscaling violation at both finite temperature and finite charge density. Using this background we explore certain features of these theories via AdS/CFT correspondence. Finally, we discuss embeddings based on the well-known sphere reductions of ten and eleven-dimensional supergravity.

Twisted backgrounds, PP-waves and nonlocal field theories

We study partially supersymmetric plane-wave like deformations of string theories and M-theory on brane backgrounds. These deformations are dual to nonlocal field theories. We calculate various expectation values of configurations of closed as well as open Wilson loops and Wilson surfaces in those theories. We also discuss the manifestation of the nonlocality structure in the supergravity backgrounds. A plane-wave like deformation of little string theory has also been studied.

The pp-wave limits of orbifolded AdS5×S5

Abstract Using the supergravity solution of N1 D3-branes probing AN2−1 singularities we study the pp-wave limit of AdS5×S5/ZN2. We show that there are two different pp-wave limits. One is the orbifold of the pp-wave limit of AdS5×S5. In this case there is no symmetry enhancement. The other case is the same as the pp-wave limit of AdS5×S5 and therefore we again see the maximal supersymmetry. We will also identify operators in the four-dimensional N =2 SU(N1)N2 gauge theory which correspond to stringy excitations in the orbifolded pp-wave geometry. The existence of the maximal pp-wave geometry indicates that there is a subsector of the corresponding N =2 gauge theories which has enhanced N =4 supersymmetry. We also study the pp-wave limits of AdS7,4×S4,7/ZN.

The dS/dS Correspondence

We present a holographic duality for the de Sitter static patch which consolidates basic features of its geometry and the behavior of gravity and brane probes, valid on timescales short compared to the decay or Poincare recurrence times. Namely de Sitter spacetime dSd(R) in d dimensions with curvature radius R is holographically dual to two conformal field theories on dSd−1(R), cut off at an energy scale 1/R where they couple to each other and to d − 1 dimensional gravity. As part of our analysis, we study brane probes in de Sitter and thermal Anti de Sitter spaces, and interpret the terms in the corresponding DBI action via strongly coupled thermal field theory. This provides a dual field theoretic interpretation of the fact that probes take forever to reach a horizon in general relativity.

New attractors, entropy function and black hole partition function

By making use of the entropy function formalism we study the generalized attractor equations in the four dimensional = 2 supergravity in presence of higher order corrections. This result might be used to understand a possible ensemble one could associate to an extremal black hole.

Thermalization in Backgrounds with Hyperscaling Violating Factor

We present an analytic solution of a Vaidya-charged black hole with hyperscaling violating factor in an Einstein-Maxwell-Dilaton model, where the scalar potential is key to the existence of a solution. By making use of this result we will study the process of thermalization after a global quench in a theory whose gravitational description is provided by the resultant solution. In particular we shall probe the system by entanglement entropy and show that it exhibits certain scaling behaviours during the process.

D-Dimensional Log Gravity

We study Einstein gravity in dimensions D ≥ 4 modified by curvature squared at critical point where the theory contains only massless gravitons. We show that at the critical point a new mode appears leading to a logarithmic gravity in the theory. The corresponding logarithmic solution may provide a gravity description of logarithmic CFT in higher dimensions. We note also that for special values of the parameters of the theory, the model admits solutions with non-relativistic isometries.

On AdS/CFT of Galilean Conformal Field Theories

We study a new contraction of a d+1 dimensional relativistic conformal algebra where n+1 directions remain unchanged. For n = 0,1 the resultant algebras admit infinite dimensional extension containing one and two copies of Virasoro algebra, respectively. For 1